Decentralized Regulation And Control Strategies For Hybrid AC/DC Microgrid

With the increasing demand for energy and strict carbon emission constraints,utilizing the renewable energy,such as solar and wind energy,seems to be the only solution for the sustainable development.With the application of distributed generation,the renewable energy can be utilized and integrated into the power system.However,the inherent characteristics of renewable energy,like intermittence and volatility,will challenge the operation of the power system,and degrade its power quality.The hybrid ac/dc microgrid,which is composed by ac subgrid,dc subgrid and interlinking bidirectional power converter,has been proposed to benefit the local accommodation of renewable generation.The ac and dc distributed generators can be easily integrated into ac subgrid or dc subgrid.Besides,the ac and dc load can be directly supplied by ac and dc subgrids,reducing unnecessary ac-dc and dc-ac conversions and power loss.Compared with the single ac or dc microgrid,the topology and operation modes of hybrid ac/dc microgrid are more complicated.The traditional decentralized control can only regulate the local sources of the same subgrid.The centralized control can realize the overall regulation of the ac/dc hybrid microgrid based on fast communication,while the communication network increases the system cost,and reduces the flexibility and reliability of the system.In this paper,a comprehensive research is carried out on the decentralized regulation and control of the hybrid ac/dc microgrid,which includes voltage supporting,power coordination and economic optimization.The main work is as follows:Firstly,the voltage supporting of hybrid ac/dc microgrid is studied.With the increasing of photovoltaic(PV)sources in the dc subgrid,their maximum power output may exceed the local demand and energy storage capacity,and the consequent imbalance power may result in dc-bus overvoltage and energy storage overcharging problems.The dc-bus voltage regulation by PV sources is proposed to solve this problem based on modified model predictive control.The proposed method enables PV sources to track the maximum power,as well as serve as voltage sources for dc-bus and share the load according to PVs’ capacities,when their available power is sufficient.To ensure the uninterrupted voltages of ac and dc subgrids in each mode,an ac voltage supporting strategy based on virtual impedance,and a dc voltage supporting strategy based on ac phase angle – dc voltage inverse droop control are proposed.By combining these two strategies,the bidirectional voltage supporting control is proposed for bidirectional power converter.With the proposed bidirectional voltage supporting control,bidirectional power converter can build ac and dc voltages,and provide bidirectional voltage supporting during unintentional islanding events.Secondly,the power coordination of hybrid ac/dc microgrid is studied.Parallel connection of bidirectional power converters is an effective way to improve the coordinated power between ac and dc subgrids,however,the circulating currents will be generated between ac and dc subgrids.After the analysis of the generation mechanism of circulating currents,a decentralized circulating currents suppression strategy is proposed based on 0 axis control.Then a power coordination strategy between two subgrids is proposed to share the power fluctuation according to subgrids’ capacities in the islanded mode.With this interaction strategy,the two subgrids can support each other and the overall performance of the hybrid microgrid is improved in the islanded mode.In the grid-connected mode,the dc load can be autonomously shared by bidirectional power converter with the proposed power sharing control,which relieves the burden of the dc voltage sources and reduces the dc voltage deviation.Finally,the decentralized economic optimization strategy of hybrid ac/dc microgrid is investigated.Traditional economic operation is to solve an optimization problem,and transmit the power command to each generator via communication.With the proposed strategy in this thesis,the distributed generators can automatically optimize the output power without solving the optimal problem and communication link.For the distributed generators in the ac and dc subgrids,the nonlinear incremental cost droop is proposed to economically share the power according to their generation costs.Besides,an economic interaction power strategy is proposed to automatically optimize the power between ac and dc subgrids.The hybrid ac/dc microgrid is modeled with the proposed nonlinear control,and its stability is analyzed under different load conditions to ensure stable operation.